1. Members of the Section have created a partial bacterial artificial chromosome contig across the critical region of the cystinosis gene on chromosome 17p. They have mapped more than 25 sequence tagged sites and several candidate genes to this region. The complete contig will be sample sequenced and its contingent of expressed sequence tags, identified by sequence comparison with databanks, will be examined. These candidate genes will then be tested to determine which is the gene defective in cystinosis. 2. A human cDNA with homology to the Haemophilus influenza UDP-N-Acetylglucosamine 2-epimerase gene has been isolated. This gene is presumably mutated in sialuria, a disorder due to defective feedback inhibition of the epimerase by CMP sialic acid. 3. A 43-year old man with MAT I/III deficiency was found to be homozygous for a 539TG insertion in the MAT1A gene. His normal neurological examination and MRI indicates that truncation of the MAT1A gene product, predicted by his mutation, does not necessarily result in brain demyelination, as suggested by other investigators. 4. Forty-nine patients with Hermansky-Pudlak syndrome were examined at the NIH Clinical Center. The 25 homozygous for a 16-bp duplication in the HPS gene typical of Puerto Rican patients had more pulmonary restrictive disease than the 24 lacking this mutation. In addition, two Puerto Rican patients with documented HPS were shown to lack the 16-bp duplication and to have an entirely normal HPS gene sequence. This indicates locus heterogeneity for HPS.